In general, as a transmission mechanism for transmitting the power of an engine of a motor vehicle, there is a belt transmission mechanism in which a belt is looped between pulleys. Additionally, as a pulley structure that is used in such a belt transmission mechanism, there is known a pulley structure having a configuration for damping a change in rotation when the change in rotation is generated in one of two rotational members (for example, Patent Document 1).
As a technique of this kind, Patent Document 1 discloses a pulley including an annular pulley member, a hub structure (to which a shaft of an alternating current generator is fixed so as not to rotate relative thereto), and a coil spring that is mounted between the annular pulley member and the hub structure. According to this configuration, when a change in rotation is generated in the hub structure, the coil spring between the hub structure and the annular pulley member is elastically deformed to thereby damp the change in rotation.
However, when the natural frequency of the pulley described in Patent Document 1 above is set to be equal to or smaller than a frequency that is considered from experience to be generated by an engine revolution speed when an engine is idling, there may be a situation in which the pulley resonates when the engine starts to rotate or stops rotating. As a result, the relative distortion displacement between the annular pulley member and the hub structure is increased abruptly, and an excessive force is applied to the coil spring, leading to a problem that a failure of the coil spring itself is called for.
To cope with this problem, there has been proposed a pulley structure which adopts a spring clutch construction. As raised in Patent Document 2, for example, in this pulley structure, an end portion of a coil spring is not fixed directly to a pulley member or a hub structure. The end portion of the coil spring is elastically deformed in a radial direction and is then attached to the pulley member or the hub structure by virtue of a restoring force of the coil spring. Then, when an input torque that exceeds a frictional torque that is generated between the end portion of the coil spring and the pulley member or the hub structure is applied to the pulley member or the hub structure, a slip is cause to occur between the end portion of the coil spring and the pulley member or the hub structure so as to suppress the application of an excessive force to the coil spring, whereby the breakage of the coil spring itself can be prevented.